Zirconium is achemical element; it hassymbolZr andatomic number 40. First identified in 1789, isolated in impure form in 1824, and manufactured at scale by 1925, pure zirconium is a lustroustransition metal with a greyish-white color that closely resembleshafnium and, to a lesser extent,titanium. It is solid at room temperature,ductile,malleable andcorrosion-resistant. The namezirconium is derived from the name of the mineralzircon, the most important source of zirconium. The word is related toPersianzargun (zircon;zar-gun, "gold-like" or "as gold").[11] Besides zircon, zirconium occurs in over 140 other minerals, includingbaddeleyite andeudialyte; most zirconium is produced as a byproduct of minerals mined for titanium andtin.
Zirconium compounds have no known biological role, though the element is widely distributed in nature and appears in small quantities in biological systems without adverse effects. There is no indication of zirconium as a carcinogen. The main hazards posed by zirconium are flammability in powder form and irritation of the eyes.
Zirconium is alustrous, greyish-white, soft, ductile, malleable metal that is solid at room temperature, though it is hard andbrittle at lesser purities.[12] In powder form, zirconium is highly flammable, but the solid form is much less prone to ignition. Zirconium is highly resistant to corrosion by alkalis, acids, salt water and other agents.[13] However, it will dissolve inhydrochloric andsulfuric acid, especially whenfluorine is present.[14]Alloys withzinc aremagnetic at less than 35 K.[13]
At room temperature zirconium exhibits a hexagonally close-packed crystal structure, α-Zr, which changes to β-Zr, a body-centered cubic crystal structure, at 863 °C. Zirconium exists in the β-phase until the melting point.[16]
Naturally occurring zirconium is composed of five isotopes.90Zr,91Zr,92Zr,94Zr, and96Zr. The first four are stable, while96 is observed to decay with a half-life of 2.34×1019 years bydouble beta emission; this is theoretically possible also for94Zr. Of these natural isotopes,90Zr is the most common, making up 51.45% of all zirconium, and96Zr is the least common, comprising only 2.80%.[10]
World production trend of zirconium mineral concentrates
Zirconium has a concentration of about 130 mg/kg within theEarth's crust and about 0.026 μg/L insea water. It is the 18th most abundant element in the crust.[18] It is not found in nature as anative metal, reflecting its intrinsic instability with respect to water. The principal commercial source of zirconium iszircon (ZrSiO4), asilicate mineral,[12] which is found primarily in Australia, Brazil, India, Russia, South Africa and the United States, as well as in smaller deposits around the world.[19] As of 2013, two-thirds of zircon mining occurs in Australia and South Africa.[20] Zircon resources exceed 60 milliontonnes worldwide[21] and annual worldwide zirconium production is approximately 900,000 tonnes.[18] Zirconium also occurs in more than 140 other minerals, including the commercially useful oresbaddeleyite andeudialyte.[22]
Zirconium is relatively abundant inS-type stars, and has been detected in the sun and in meteorites. Lunar rock samples brought back from severalApollo missions to the moon have a high zirconium oxide content relative to terrestrial rocks.[23]
EPR spectroscopy has been used in investigations of the unusual 3+ valence state of zirconium. The EPR spectrum of Zr3+, which has been initially observed as a parasitic signal in Fe‐doped single crystals of ScPO4, was definitively identified by preparing single crystals of ScPO4 doped with isotopically enriched (94.6%)91Zr. Single crystals of LuPO4 and YPO4 doped with both naturally abundant and isotopically enriched Zr have also been grown and investigated.[24]
Zirconium is a by-product formed after mining and processing of thetitanium mineralsilmenite andrutile, as well astin mining.[25] From 2003 to 2007, while prices for the mineral zircon steadily increased from $360 to $840 per tonne, the price for unwrought zirconium metal decreased from $39,900 to $22,700 per ton. Zirconium metal is much more expensive thanzircon because the reduction processes are costly.[21]
Collected from coastal waters, zircon-bearing sand is purified byspiral concentrators to separate lighter materials, which are then returned to the water because they are natural components of beach sand. Usingmagnetic separation, the titanium oresilmenite andrutile are removed.[26]
Most zircon is used directly in commercial applications, but a small percentage is converted to the metal. Most Zr metal is produced by the reduction of thezirconium(IV) chloride withmagnesium metal in theKroll process.[13] The resulting metal issintered until sufficiently ductile for metalworking.[19]
Commercial zirconium metal typically contains 1–3% ofhafnium,[27] which is usually not problematic because the chemical properties of hafnium and zirconium are very similar. Their neutron-absorbing properties differ strongly, however, necessitating the separation of hafnium from zirconium for nuclear reactors.[28] Several separation schemes are in use.[27] Theliquid-liquid extraction of thethiocyanate-oxide derivatives exploits the fact that the hafnium derivative is slightly more soluble inmethyl isobutyl ketone than in water. This method accounts for roughly two-thirds of pure zirconium production,[29] though other methods are being researched;[30] for instance, in India, a TBP-nitrate solvent extraction process is used for the separation of zirconium from other metals.[31] Zr and Hf can also be separated byfractional crystallization of potassium hexafluorozirconate (K2ZrF6), which is less soluble in water than the analogous hafnium derivative.Fractional distillation of the tetrachlorides, also calledextractive distillation, is also used.[30][32]
Vacuumarc melting, combined with the use of hot extruding techniques andsupercooled copper hearths, is capable of producing zirconium that has been purified of oxygen, nitrogen, and carbon.[33]
Hafnium must be removed from zirconium for nuclear applications because hafnium has a neutron absorption cross-section 600 times greater than zirconium.[34] The separated hafnium can be used for reactorcontrol rods.[35]
Like othertransition metals, zirconium forms a wide range ofinorganic compounds andcoordination complexes.[36] In general, these compounds are colourless diamagnetic solids wherein zirconium has theoxidation state +4. Some organometallic compounds are considered to have Zr(II) oxidation state.[6] Non-equilibrium oxidation states between 0 and 4 have been detected during zirconium oxidation.[7]
The most common oxide iszirconium dioxide, ZrO2, also known aszirconia. This clear to white-coloured solid has exceptionalfracture toughness (for a ceramic) and chemical resistance, especially in itscubic form.[37] These properties make zirconia useful as athermal barrier coating,[38] although it is also a commondiamond substitute.[37] Zirconium monoxide, ZrO, is also known andS-type stars are recognised by detection of its emission lines.[39]
Zirconium tungstate has the unusual property of shrinking in all dimensions when heated, whereas most other substances expand when heated.[13]Zirconyl chloride is one of the few water-soluble zirconium complexes, with the formula [Zr4(OH)12(H2O)16]Cl8.[36]
Zirconium carbide andzirconium nitride are refractory solids. Both are highlycorrosion-resistant and find uses in high-temperature resistant coatings and cutting tools.[40] Zirconium hydride phases are known to form when zirconium alloys are exposed to large quantities ofhydrogen over time; due to the brittleness of zirconium hydrides relative to zirconium alloys, the mitigation of zirconium hydride formation was highly studied during the development of the first commercialnuclear reactors, in which zirconium carbide was a frequently used material.[41]
All four common halides are known,ZrF4,ZrCl4,ZrBr4, andZrI4. All have polymeric structures and are far less volatile than the corresponding titanium tetrahalides; they find applications in the formation of organic complexes such aszirconocene dichloride.[43] All tend tohydrolyse to give the so-called oxyhalides and dioxides.[27]
Fusion of the tetrahalides with additional metal gives lower zirconium halides (e.g.ZrCl3). These adopt a layered structure, conducting within the layers but not perpendicular thereto.[44]
The corresponding tetraalkoxides are also known. Unlike the halides, the alkoxides dissolve in nonpolar solvents. Dihydrogen hexafluorozirconate is used in the metal finishing industry as an etching agent to promote paint adhesion.[45]
The zirconium-containing mineral zircon and related minerals (jargoon,jacinth, or hyacinth,ligure) were mentioned in biblical writings.[13][28] The mineral was not known to contain a new element until 1789,[49] whenKlaproth analyzed a jargoon from the island of Ceylon (nowSri Lanka). He named the new element Zirkonerde (zirconia),[13] related to thePersianzargun (zircon;zar-gun, "gold-like" or "as gold").[11]Humphry Davy attempted to isolate this new element in 1808 throughelectrolysis, but failed.[12] Zirconium metal was first obtained in an impure form in 1824 byBerzelius by heating a mixture of potassium and potassium zirconium fluoride in an iron tube.[13]
Approximately 900,000 tonnes of zirconium ores were mined in 1995, mostly as zircon.[27]
Most zircon is used directly in high-temperature applications. Because it is refractory, hard, and resistant to chemical attack, zircon finds many applications. Its main use is as an opacifier, conferring a white, opaque appearance to ceramic materials. Because of its chemical resistance, zircon is also used in aggressive environments, such as moulds for molten metals.[27]
Zirconium dioxide (ZrO2) is used in laboratory crucibles, in metallurgical furnaces, and as a refractory material[13] Because it is mechanically strong and flexible, it can besintered intoceramic knives and other blades.[51] Zircon (ZrSiO4) andcubic zirconia (ZrO2) are cut into gemstones for use in jewelry. Zirconium dioxide is a component in someabrasives, such as grinding wheels andsandpaper.[49] Zircon is also used indating of rocks from about the time of the Earth's formation through the measurement of its inherentradioisotopes, most oftenuranium andlead.[52]
A small fraction of the zircon is converted to the metal, which finds various niche applications. Because of zirconium's excellent resistance to corrosion, it is often used as an alloying agent in materials that are exposed to aggressive environments, such as surgical appliances, light filaments, and watch cases. The high reactivity of zirconium with oxygen at high temperatures is exploited in some specialised applications such as explosive primers and asgetters invacuum tubes.[53] Zirconium powder is used as a degassing agent in electron tubes, while zirconium wire and sheets are utilized for grid andanode supports.[54][55] Burning zirconium was used as a light source in somephotographic flashbulbs. Zirconium powder with amesh size from 10 to 80 is occasionally used in pyrotechnic compositions to generatesparks. The high reactivity of zirconium leads to bright white sparks.[56]
Cladding for nuclear reactor fuels consumes about 1% of the zirconium supply,[27] mainly in the form ofzircaloys. The desired properties of these alloys are a low neutron-capturecross-section and resistance to corrosion under normal service conditions.[19][13] Efficient methods for removing the hafnium impurities were developed to serve this purpose.[28]
One disadvantage of zirconium alloys is the reactivity with water, producinghydrogen, leading to degradation of thefuel rod cladding:[57]
Zr + 2 H2O → ZrO2 + 2 H2
Hydrolysis is very slow below 100 °C, but rapid at temperature above 900 °C. Most metals undergo similar reactions. The redox reaction is relevant to the instability offuel assemblies at high temperatures.[58] This reaction occurred in the reactors 1, 2 and 3 of theFukushima I Nuclear Power Plant (Japan) after the reactor cooling was interrupted by theearthquake and tsunami disaster of March 11, 2011, leading to theFukushima I nuclear accidents. After venting the hydrogen in the maintenance hall of those three reactors, the mixture of hydrogen with atmosphericoxygen exploded, severely damaging the installations and at least one of the containment buildings.[59]
Materials fabricated from zirconium metal and ZrO2 are used in space vehicles where resistance to heat is needed.[28]
High temperature parts such as combustors, blades, and vanes injet engines and stationarygas turbines are increasingly being protected by thinceramic layers and/or paintable coatings, usually composed of a mixture of zirconia andyttria.[61]
Zirconium-bearing compounds are used in many biomedical applications, including dental implants andcrowns, knee and hip replacements, middle-earossicular chain reconstruction, and other restorative andprosthetic devices.[64]
Zirconium bindsurea, a property that has been utilized extensively to the benefit of patients withchronic kidney disease.[64] For example, zirconium is a primary component of thesorbent column dependent dialysate regeneration and recirculation system known as the REDY system, which was first introduced in 1973. More than 2,000,000dialysis treatments have been performed using the sorbent column in the REDY system.[65] Although the REDY system was superseded in the 1990s by less expensive alternatives, new sorbent-based dialysis systems are being evaluated and approved by the U.S.Food and Drug Administration (FDA). Renal Solutions developed the DIALISORB technology, a portable, low water dialysis system. Also, developmental versions of a Wearable Artificial Kidney have incorporated sorbent-based technologies.[66]
Although zirconium has no known biological role, the human body contains, on average, 250 milligrams of zirconium, and daily intake is approximately 4.15 milligrams (3.5 milligrams from food and 0.65 milligrams from water), depending on dietary habits.[69] Zirconium is widely distributed in nature and is found in all biological systems, for example: 2.86 μg/g in whole wheat, 3.09 μg/g in brown rice, 0.55 μg/g inspinach, 1.23 μg/g in eggs, and 0.86 μg/g in ground beef.[69] Further, zirconium is commonly used in commercial products (e.g.deodorant sticks, aerosolantiperspirants) and also in water purification (e.g. control ofphosphorus pollution, bacteria- and pyrogen-contaminated water).[64]
Short-term exposure to zirconium powder can cause irritation, but only contact with the eyes requires medical attention.[70] Persistent exposure tozirconium tetrachloride results in increased mortality in rats and guinea pigs and a decrease of bloodhemoglobin andred blood cells in dogs. However, in a study of 20 rats given a standard diet containing ~4% zirconium oxide, there were no adverse effects on growth rate, blood and urine parameters, or mortality.[71] The U.S.Occupational Safety and Health Administration (OSHA) legal limit (permissible exposure limit) for zirconium exposure is 5 mg/m3 over an 8-hour workday. TheNational Institute for Occupational Safety and Health (NIOSH)recommended exposure limit (REL) is 5 mg/m3 over an 8-hour workday and a short term limit of 10 mg/m3. At levels of 25 mg/m3, zirconium isimmediately dangerous to life and health.[72] However, zirconium is not considered an industrial health hazard.[64] Furthermore, reports of zirconium-related adverse reactions are rare and, in general, rigorous cause-and-effect relationships have not been established.[64] No evidence has been validated that zirconium is carcinogenic[73] or genotoxic.[74]
Among the numerous radioactive isotopes of zirconium,93Zr is among the most common. It is released as aproduct of nuclear fission of235U and239Pu, mainly in nuclear power plants and during nuclear weapons tests in the 1950s and 1960s. It has a very long half-life (1.53 million years), its decay emits only low energy radiations, and it is not considered particularly hazardous.[75]
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